What is oscilloscope bandwidth?
An oscilloscope is an electronic device that plots a two-dimensional representation of signals as a function of time and can be used to test and observe the variation in voltage signals over time.
The oscilloscope can be used to measure other signals like sounds, amplitude, rise time, distortion, vibrations, time interval, frequency, and some others, but the device converts the test results to voltage before they are displayed in waveforms.
One of the important feature you would consider when getting an oscilloscope for your projects is the bandwidth of the device. The bandwidth of an oscilloscope determines its ability to measure a signal. It specifies the accuracy and range of frequency the oscilloscope can measure in Hertz.
Introduction to bandwidth in an oscilloscope
All test equipment would have a range of frequency they can respond to, that’s where the bandwidth comes in; it determines the measurement limits of the device.
The bandwidth of an oscilloscope is 70.7% of the original signal, which is given as a 3 decibel down point. Once the scope receives an input frequency that is above the 0.707 at direct current and or the lowest alternating current frequency at 3 decibels, the response of the device would drop.
What causes this attenuation and down point is the combined effect of a series of a phenomenon that rises at a higher frequency which is the inductive reactance, or a parallel phenomenon that reduces at higher frequencies which is the capacitive reactance.
When measuring frequency, an increase would result to a reduced capability of measuring accuracy displayed by the oscilloscope because as the frequency rises, the inductive and capacitive reactance in combination would increase impedance and limit the bandwidth.
With a higher bandwidth would give a faster result, the details of your waveform would be safer, and you would have a correct measurement of amplitude. So in an actual sense, if you are selecting your oscilloscope based on bandwidth, getting a 50 MHz would be better than a 20 MHz, and a 100 MHz is better than a 50 MHz
Also note that when your bandwidth is too much your oscilloscope would capture unnecessary noises that would also cause inaccurate measurements, where the details of amplitude measured in waveform to be incorrect, you could also lose some crucial information. The correct range for an oscilloscope bandwidth should fall between zero and -3 decibels.
Let’s explain further!
What happens if the bandwidth is too small?
When the bandwidth of your oscilloscope is small, and the signal you are measuring has a higher frequency than the cutoff, there would be a distorted version, and an attenuation of signals or no signal might be detected at all.
Also, using an oscilloscope to measure a signal with the same speed as your bandwidth is not correct. For example, using a 100 MHz oscilloscope to measure a 100 MHz signal is not a good idea.
For accurate voltage measurement, the noticeable effect of the bandwidth would only be 20 percent of the total cut off, so you should instead use a digital measurement with oscilloscopes five times higher than the signal or take analog signal measurements with oscilloscope three times higher to avoid distorted results.
What happens if the bandwidth is too much?
The solution to a limited bandwidth is not getting an oscilloscope with the highest possible bandwidth, too much bandwidth have problems of their own; a higher bandwidth than its required would create glitches due to the environmental noises they capture.
Noise occurs at all frequencies, so regardless of the frequency of your signal there would be equal noise to match it, like having a 350 MHz oscilloscope would show noise up to 350 MHz all through the entire measurement.
Apart from the environmental noises, the filtering, processing and digitizing of the scope would also add noise to your signals, although some high-quality oscilloscopes are built to reduce these noises still, they cannot entirely remove the noise.
An increased bandwidth measuring a lower signal would only increase the noise, for example, using 350 MHz scope to measure a 100 MHz signal gives enough chances for accuracy and reduce the attenuation and distortion, but it would only add a higher frequency noise content to your readings.
Some oscilloscope includes a narrow tolerance range, so when you measure low frequencies with high bandwidth oscilloscope, for cleaner measurement, you can turn on the hardware filter in the oscilloscope to eliminate the frequency noise.
A reason to consider the bandwidth an oscilloscope is an effect on the price; the price of an oscilloscope is always rated by the number of the analog sample by seconds that it reads and the bandwidth.
An oscilloscope with higher bandwidth is always more expensive, but it is better to get an oscilloscope that can allow you upgrade the bandwidth with a software, so when you need an increase, so you would only have to make an upgrade, not a new purchase.
Another related factor that makes the bandwidth a priority when purchasing an oscilloscope is the rise time (the duration of the resolvable fastest pulse), which is approximately 0.35 divided by the bandwidth in Hertz.